A San Francisco man is under arrest in connection with a bizarre incident at the city’s Main Library branch in which he struck another patron over the head with a chair in an apparently unprovoked attack.

KCBS 5 reports Clifton Moore inexplicably grew enraged and hit a man over the head with a chair Wednesday at the city library system’s Main Branch, located across from City Hall.

Moore, who was quickly detained by library security and ultimately arrested, was charged with two counts of assault with a deadly weapon and aggravated assault, according to the San Francisco Examiner.

The strange sequence reportedly unfolded when Moore, 25, whacked an unidentified 61-year-old man who was using a computer on the library’s first floor.

The bloodied victim later told police he did not know Moore, could not identify him and wasn’t sure if he briefly lost consciousness during the ordeal, according to the Examiner.

The victim was taken to an area hospital and treated for non-life-threatening injuries, as other library patrons fingered Moore for security and police, the paper writes.

The Examiner also reports that such examples of bad behavior are rather common at the city library system’s 28 branches.

Several library employees, who asked to remain anonymous, told the paper that such incidents happen often and that although security did its job, they are still understaffed.

In the wake of the attack, library officials rushed to reassure patrons that the institution is, indeed, safe.

“This is, you know, a very urban neighborhood, certainly. But the library is a very democratic institution,” Michelle Jeffers, a library spokeswoman, reportedly said. “Possibly the greatest democratic institution and we are open to everyone.”

A San Francisco police sergeant will reportedly join 16 security officers at the main branch on Monday, although officials told KCBS 5 the assignment was made before Wednesday’s unsettling attack.